Examples of known solvents used in the preparation of an oxide crystal film with a Y123 type crystal structure on top of a substrate, for example in a process in which a YBa2Cu3O6+d crystal film (hereafter abbreviated as a “YBCO film”) is grown from a solution by a liquid phase method (such as a crystal pulling type method using a solution), include BaO—CuO, BaO—CuO—BaF2, and BaO—CuO—BaF2—Ag (see “Supercond. Sci. Technol., 13(2000)” pp 82–87, printed in the U.K.).
With conventional solvent compositions and atmospheric conditions, the minimum temperature for film formation was limited to approximately 860° C. (see the above reference).
In film formation from a solution phase using a liquid phase method, typically a seed crystal film must first be formed on the surface of the substrate. This seed crystal film acts as a seed for crystal film growth, and also performs the function of protecting the substrate from the highly reactive solution. However, the inventors of the present invention have discovered that if the film formation temperature is too high, the seed crystal can break down, and in severe cases, the substrate being used can react with the solution, making the growing and preparation of a high quality crystal film on the substrate impossible.
For example, in YBCO film formation using an Ag—Cu alloy substrate, when the solution temperature was 920° C., a reaction layer between the substrate and the solution developed between the YBCO film and the substrate, and this reaction layer caused the YBCO film to be more prone to cracking. At solution temperatures exceeding 860° C., for example during film formation at 880° C., almost no reaction layer developed, but regions in which the YBCO film did not form were observed.
When a Ni substrate that had been covered with a nickel oxide (NiO) film by surface oxidation was used, film formation at a set solution temperature of either 920° C. or 880° C. resulted in a partial formation of a reaction layer between the substrate and the solution, although this reaction layer formation was less pronounced in the film formation at 880° C.
When an oxide substrate comprising a polycrystalline yttria stabilized zirconia (YSZ) was used, for film formation at temperatures in the vicinity of 880° C., reaction between the substrate and the solution resulted in film separation if the film formation process was conducted over an extended time period. Furthermore, when Hastelloy (a brand name of Haynes Stellite Inc., a Ni based corrosion resistant alloy) with an oriented YSZ intermediate layer was used as a substrate, cracks developed in the YBCO film with a set solution temperature of 860° C.